A significant amount of design and manufacturing effort goes into the shape and configuration of the wings used for commercial transport aircraft. The wings must meet a myriad of design goals, including producing high lift with low drag, and providing sufficient structure to carry a payload, without contributing unnecessarily to aircraft weight. To meet these often contradictory design requirements, designers have developed a number of techniques for distributing the load over the span of the wing in a manner that produces sufficient lift without requiring unnecessary structure. For example, the “ideal” load distribution for a flat wing is generally elliptical. However, conventional aircraft wings are typically not designed with elliptical span loads. Instead, they are designed with compromised “triangular” span loads that reduce structural bending loads at the root of the wing. Such designs trade a slight increase in induced drag for a reduction in airframe weight. The degree of compromise varies considerably from one aircraft to another.
To produce such a triangular span load, the wing tip is typically twisted to produce “washout.” Washout refers to a wing tip that is twisted so that the leading edge moves downward and the trailing edge moves upward relative to the orientations of these components at the wing root. Washing out the wing tip in this manner lowers the angle of attack of the wing tip with respect to the wing root, thereby reducing the lift distribution toward the wing tip. This arrangement also prevents the entire wing from stalling all at once at high angles of attack. Instead, the wing root tends to stall first and the wing tip tends to stall last because it is at an effectively lower angle of attack.
To recapture some of the wing loading lost by washing out the wing tip, aircraft manufacturers have added wing tip devices to the outboard regions of the wings. For example, FIG. 1 illustrates a Boeing 747 aircraft 10 having winglets 21 mounted at the tips of the wings 20. The winglets 21 can delay the gradual spanwise reduction in lift toward the tip of the wing 20, thereby increasing the overall lift of the wing 20 without increasing the wing span. While such devices have proven effective in aircraft design, there is a continual need to improve the effectiveness and/or efficiency of wing designs, thereby improving overall aircraft performance and/or fuel economy.